Electroless surface treatment plated layers of printed circuit board and method for preparing the same

ABSTRACT

An electroless surface treatment plated layer of a printed circuit board, a method for preparing the same, and printed circuit board including the same. The electroless surface treatment plated layer includes: electroless nickel (Ni) plated coating/palladium (Pd) plated coating/gold (Au) plated coating, wherein the electroless nickel, palladium, and gold plated coatings have thicknesses of 0.02 to 1 μm, 0.01 to 0.3 μm, and 0.01 to 0.5 μm, respectively. In the electroless surface treatment plated layer of the printed circuit board, a thickness of the nickel plated coating is specially minimized to 0.02 to 1 μm, thereby making it possible to form an optimized electroless Ni/Pd/Au surface treatment plated layer.

CROSS REFERENCE(S) TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.13/532,063, filed Jun. 25, 2012, which claims the benefit under 35U.S.C. Section 119 of Korean Patent Application Serial No.10-2011-0062940, entitled “Electroless Surface Treatment Plated Layersof Printed Circuit Board and Method for Preparing the Same” filed onJun. 28, 2011, which is hereby incorporated by reference in its entiretyinto this application.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an electroless surface treatment platedlayer of a printed circuit board, a method for preparing the same, and aprinted circuit board including the same.

2. Description of the Related Art

In accordance with an increase in density of an electronic component,various technologies regarding surface treatment of a printed circuitboard (PCB) have been developed. In accordance with the recent demand ofPCB products that have been thinned and densified, a process of treatinga surface of the PCB has been recently changed from electro Ni/Ausurface treatment into electroless surface treatment in which taillessimplementation may be easily performed in order to simplify a processand solve a problem such as noise, or the like.

Particularly, since the existing used electroless Ni/Au (hereinafter,referred to as ENIG) and recently prominent electroless Ni/Pd/Au(hereinafter, referred to as ENEPIG) have excellent solder connectionreliability and wire bonding reliability, they have been used in variousfields as well as a package substrate. However, in accordance withminiaturization of a wiring for increasing density of the electroniccomponent, problems of a technology of surface-treating the circuitboard according to the related art have started to emerge.

Meanwhile, as a scheme of connecting various devices such as a die, amain board, and the like, to each other, there are mainly two schemes,that is, a wire-bonding scheme and a solder joint scheme.

FIGS. 1A and 1B are, respectively, a cross-sectional view and a top viewwhen a plated layer and a device are connected to each other using thewire-bonding scheme according to the related art. Referring to FIGS. 1Aand 1B, a polymer resin layer 20 is first formed at a portion except fora copper layer 10 in a printed circuit board to thereby subsequentlyserve as a resist to plating, and electroless Ni layer 31/Pd layer 32/Aulayer 33 are formed as a plated layer 30 using an electroless platingmethod in order to protect the copper layer 10. In addition, after theelectroless Ni/Pd/Au surface treatment plated layer 30 is formed, themetal layers are interconnected using a gold (Au) wire, or the like.

In a substrate including the plated layer 30 of the ENIG or ENEPIG, inthe case of the ENIG, the Ni layer has a thickness of at least 3 μm andthe Au layer has a thickness of 0.05 to 0.5 μm, and in the case of theENEPIG, the Ni layer has a thickness of at least 3 μm, the Pd layer hasa thickness of 0.05 to 0.3 μm, and the Au layer has a thickness of 0.05to 0.5 μm, as seen in FIG. 2.

That is, the Ni layer generally has the thickest thickness of 3 μmormore, more specifically, 3 to 7 μm. The reason is that the electrolessNi layer needs to have coating performance without a defect in order toserve as a barrier layer to thereby suppress diffusion of underlyingcopper.

However, when the thickness of the Ni layer is thick, as a frequencyincreases, a phenomenon in which a current flows in a surface due to askin effect, such that it is concentrated on an outermost layer of Ni/Auor Ni/Pd/Au rather than an inner layer of Cu wiring occurs. However, theNi layer has electrical resistance higher than that of CU, such thatelectrical characteristics are deteriorated. Therefore, a solutionthereof has been demanded.

Further, in the case in which the layers have the above-mentionedthicknesses, it is impossible to reduce a space between patterns to 25μm or less, which is an object of the next generation technology. Thereason is that when the space between the patterns becomes narrow, theNi layer abnormally grows between the patterns to thereby cause anelectrical bridge.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electroless surfacetreatment plated layer of a printed circuit board capable of solving anon-uniformity problem of current flow caused by a skin effect bysolving several problems generated due to a thick thickness of a nickellayer included in a plated layer in connecting the surface treatmentplated layer and external devices to each other using a wire bondingscheme.

Another object of the present invention is to provide a method forforming an electroless surface treatment plated layer of a printedcircuit board in which a plated layer is formed on a surface of a copperlayer in order to protect the copper layer.

Another object of the present invention is to provide a printed circuitboard including an electroless surface treatment plated layer.

According to an exemplary embodiment of the present invention, there isprovided an electroless surface treatment plated layer of a printedcircuit board, the electroless surface treatment plated layer including:electroless nickel (Ni) plated coating/palladium (Pd) platedcoating/gold (Au) plated coating, wherein the electroless nickel,palladium, and gold plated coatings have thicknesses of 0.02 to 1 μm,0.01 to 0.3 μm, and 0.01 to 0.5 μm, respectively.

The electroless surface treatment plated layer may be connected toexternal devices in a wire bonding scheme.

8 wt % or more of phosphorus (P) may be contained in the electrolessnickel plated coating.

The phosphorus (P) may be to prevent oxidation of the electroless nickelplated coating.

According to another exemplary embodiment of the present invention,there is provided a printed circuit board including the electrolesssurface treatment plated layer as described above.

As a frequency band increases, due to a skin effect phenomenon in whicha current flows along a conductor surface, the current flows in a nickellayer having high electrical specific resistance at the time of use ofan existing ENEPIG layer, such that an electrical resistance valueincreases. However, in the case of the printed circuit board accordingto the present invention including the electroless surface treatmentplated layer as described above, a thickness of plated layer in theelectroless surface treatment plated a nickel layer is significantlyreduced to alleviate an increase in electrical resistance value in ahigh frequency band, thereby making it possible to improve an electricalsignal.

Plated layers formed as the electroless surface treatment plated layerof the printed circuit board may have a space of 25 μm or lesstherebetween.

According to another exemplary embodiment of the present invention,there is provided a method for forming an electroless plated layer of aprinted circuit board, the method including: forming an electrolesssurface treatment plated layer by sequentially forming nickel (Ni)plated coating/palladium (Pd) plated coating/gold (Au) plated coating,the electroless nickel, palladium, and gold plated coatings havethicknesses of 0.02 to 1 μm, 0.01 to 0.3 μm, and 0.01 to 0.5 μm,respectively.

8 wt % or more of phosphorus (P) may be contained in the electrolessnickel plated coating.

A phosphorus content in the electroless nickel plated coating may beadjusted to the above-mentioned by relatively lowering a pH of a Niplating solution at the time of electroless nickel plating. That is,although a pH adjustment range may be slightly different according to acommonly used electroless nickel plating solution, in the case of thewidely used NPR 4 agent available from Uyemora & Co., Ltd, the pH ismanaged to be 4.5 or less or concentration of a sulfur compound additiveis appropriately lowered, thereby making it possible to adjust the pH tobe in the above-mentioned range.

A phosphorus content in the electroless nickel plated coating may beadjusted by adding an organic compound additive to a nickel platingsolution.

The electroless gold plated coating may be formed in asubstitution/reduction type or a substitution type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are, respectively, a cross-sectional view and a top viewwhen a plated layer and a device are connected to each other using thewire-bonding scheme according to the related art;

FIG. 2 is a scanning electron microscope (SEM) photograph of electrolessNi/Pd/Au surface treatment metal layer formed on a Cu layer according tothe related art;

FIG. 3 is a view showing a structure of electroless Ni/Pd/Au surfacetreatment metal layer according to an exemplary embodiment of thepresent invention; and

FIG. 4 is a SEM photograph of electroless Ni/Pd/Au surface treatmentmetal layer formed on a Cu layer according to an exemplary embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in more detail.

Terms used in the present specification are for explaining theembodiments rather than limiting the present invention. Unlessexplicitly described to the contrary, a singular form includes a pluralform in the present specification. The word “comprise” and variationssuch as “comprises” or “comprising,” will be understood to imply theinclusion of stated constituents, steps, operations and/or elements butnot the exclusion of any other constituents, steps, operations and/orelements.

The present invention relates to an electroless surface treatment platedlayer of a printed circuit board including nickel (Ni) platedcoating/palladium (Pd) plated coating/gold (Au) plated coating formed ona connection terminal in an electroless plating scheme, wherein theelectroless nickel, palladium, and gold plated coatings have thicknessesof 0.02 to 1 μm, 0.01 to 0.3 μm, and 0.01 to 0.5 μm, respectively.

FIG. 3 shows a structure of electroless Ni/Pd/Au surface treatmentplated layer of a printed circuit board according to an exemplaryembodiment of the present invention. Referring to FIG. 3, a polymerresin layer 120 is formed at a portion except for a copper layer 110 ina printed circuit board to thereby subsequently serve as a resist toplating, and electroless Ni plated coating 131/Pd plated coating 132/Auplated coating 133 are formed as a plated layer 130 using an electrolessplating method in order to protect the copper layer 110.

According to the exemplary embodiment of the present invention, in orderto overcome a disadvantage of the scheme according to the related art, athickness of the nickel plated coating 131 is reduced, thereby making itpossible to maximally suppress an increase in resistance generated atthe time of an increase in frequency, which is a problem according tothe related art.

The nickel plated coating 131 of the electroless surface treatmentplated layer according to the exemplary embodiment of the presentinvention may have a thickness of 0.02 to 1 μm. When the nickel platedcoating has a thickness exceeding 1 μm, a Ni layer having highelectrical resistance has a relatively thick thickness, such that animprovement effect of electrical characteristics is low in a highfrequency band. In addition, when the nickel plated coating has athickness less than 0.02 μm, a problem such as deterioration ofcorrosion resistance, or the like, is generated.

After the electroless nickel plated coating 131 is formed, theelectroless palladium (Pd) plated coating 132 is plated on the nickelplated coating. The electroless Pd plated coating 132 serves to suppressa phenomenon in which an electrochemically active Ni layer is reactedand corroded with Au at the time of immersion Au or electroless Auplating. The electrode palladium plated coating according to theexemplary embodiment of the present invention may have a thickness of0.01 to 0.3 μm. When the electroless palladium (Pd) plated coating has athickness exceeding 0.3 μm, the cost increases.

Finally, in the electroless surface treatment plated layer, after theelectroless palladium plated coating 132 is formed, the electroless gold(Au) plated coating 133 is plated on the electroless palladium platedcoating. The electroless Au plated coating 133 serves to preventoxidation to thereby maintain wire bonding characteristics. Theelectroless gold plated coating according to the exemplary embodiment ofthe present invention may have a thickness of 0.01 to 0.5 μm. When theelectroless Au plated coating has a thickness exceeding 0.5 μm, the costincreases.

The electroless Ni/Pd/Au surface treatment plated layer 130 formed asdescribed above may be connected to external devices in a wire bondingscheme using a gold (Au) wire.

In addition, 8 wt % or more of phosphorus, and preferably, 9 to 11 wt %of phosphorus is contained in the electroless nickel plated coating 131.The phosphorus is to prevent oxidation of the electroless nickel platedcoating. When relatively high wt % of phosphorus is contained in theelectroless nickel plated coating, the oxidation of the electrolessnickel plated coating may be effectively prevented.

According to an exemplary embodiment of the present invention, a printedcircuit board including the electroless surface treatment plated layermay be provided.

In the case of the printed circuit board including the electrolesssurface treatment plated layer according to the exemplary embodiment ofthe present invention, even though a frequency band increases, anelectrical resistance value of the electroless surface treatment platedlayer does not increase. The thickness of the nickel layer forming theelectroless surface treatment plated layer is minimized, thereby makingit possible to basically solve a problem in which even through thefrequency band increases, a current flows in the surface treatmentplated layer. Therefore, a problem in which the flow of the current isconcentrated only on the surface layer caused by the skin effect in therelated art is solved, thereby making it possible to allow the currentto uniformly flow.

Further, embodiment of in the printed circuit board according to theexemplary the present invention, plated layers formed as the electrolesssurface treatment plated layer may have a space of 25 μm or lesstherebetween.

A method for forming an electroless surface treatment plated layer of aprinted circuit board according to an exemplary embodiment of thepresent invention will be described. The method for forming anelectroless surface treatment plated layer of a printed circuit boardincludes forming an electroless surface treatment plated layer bysequentially forming nickel (Ni) plated coating/palladium (Pd) platedcoating/gold (Au) plated coating, wherein the electroless nickel,palladium, and gold plated coatings have thicknesses of 0.02 to 1 μm,0.01 to 0.3 μm, and 0.01 to 0.5 μm, respectively.

Nickel, palladium, and gold plating solutions configuring theelectroless surface treatment plated layer according to the exemplaryembodiment of the present invention is not specifically limited but maybe any solution generally used in the art. In addition, a specificplating method is also not specifically limited but conforms to ageneral level.

However, the electroless surface treatment plated layer according to theexemplary embodiment of the present invention needs to be plated so thatthe electroless nickel, palladium, and gold plated coatings havethicknesses of 0.02 to 1 μm, 0.01 to 0.3 μm, and 0.01 to 0.5 μm,respectively.

Particularly, as the nickel plating solution, hypophosphite is generallyused, and phosphorus is contained in a precipitated coating. Thephosphorus (P) may effectively serve to prevent the oxidation of the Niplated coating. Therefore, 8 wt % or more of phosphorus (P), andpreferably, 9 to 11 wt % of phosphorus (P) is contained in theelectroless nickel plated coating 131 according to the exemplaryembodiment of the present invention.

Two methods may be used in order to allow the electroless nickel platedcoating according to the exemplary embodiment of the present inventionto contain the above-mentioned phosphorus content. One method is amethod of lowering a pH of the electroless nickel plating solution. Inthe case of the widely used NPR-4 agent available from Uyemora & Co.,Ltd, a nickel plating solution has a pH of 4.5 to 4.7. However, in thecase of the present invention, the pH of the nickel plated coated isreduced to 4.2 to 4.4 to thereby maintain a phosphorus content at arelatively high level.

The other method is a method of reducing a content of a sulfur compoundadditive added to the electroless nickel plating solution to therebycontrol a phosphorus content in the electroless nickel plated coating tobe in the above-mentioned range. That is, the content, or the like, ofthe sulfur compound such as Thio Urea is lowered, thereby making itpossible to allow the electroless nickel plated coating to have adesired level of phosphorus content.

According to the exemplary embodiment of the present invention, theelectroless gold plated coating may be formed in asubstitution/reduction type. In addition, a substitution type ofimmersion gold plated coating layer may be formed. When the electrolessgold plated coating is formed in a general substitution type, acorrosion hole is formed in densely formed electroless Ni and Pd platedcoatings to thereby deteriorate the ability to prevent the diffusion ofthe copper. However, when the electroless gold plated coating is formedin the substitution/reduction type, a substitution reaction is directlyperformed in an early stage of a reaction. Therefore, the Ni and Pdplated coatings are not attacked, thereby making it possible to obtain aplated layer having a dense structure. However, a substitution type ofAu, which may be easily managed and is inexpensive, may also be used.

Hereinafter, an example of the present invention will be described indetail with reference to the accompanying drawings. However, thisexample is only to illustrate the present invention and is not to beconstrued as limiting a scope of the present invention.

EXAMPLE 1

1) Electroless Ni Plating

A substrate subjected to pretreatment was immersed in an electroless Niplating solution (pH: 4.2 to 4.4, NPR-4: a product available fromUyemora & Co., Ltd) allowing 10 wt % of phosphorus to be contained in aplated coating at a temperature of 65 t for 1 minute and then cleanedfor 2 minutes to thereby obtain an electroless nickel plated coatinghaving a thickness of 0.1 μm.

2) Electroless Pd Plating

The substrate subjected to the electroless Ni plating was immersed inXTP (pH: 7.2, a product available from Uyemora & Co., Ltd), which is anelectroless Pd plating solution, at a temperature of 50° C. for 10minutes and then cleaned for 2 minutes to thereby obtain an electrolesspalladium plated coating having a thickness of 0.1 μm.

3) Electroless Au Plating

The substrate subjected to the electroless Ni plating and theelectroless Pd plating was immersed in an electroless gold platingsolution (GoBright TSB-72, a product available from Uyemora & Co., Ltd)at a temperature of sot for 5 minutes, cleaned for 2 minutes, and thendried at a temperature of 150 t for 5 minutes by a ventilation drier tothereby obtain electroless nickel/palladium/gold plated layer in whichan electroless gold plated coating having a thickness of 0.1 μm isformed.

4) Wire Bonding

The electroless nickel/palladium/gold surface treatment plated layer andexternal devices were connected to each other by a gold wire.

COMPARATIVE EXAMPLE 1

Electroless nickel/palladium/gold plated layer was obtained through thesame process as that of Example 1 except that an electroless nickelplated coating having a thickness of 5 μm is formed.

EXPERIMENTAL EXAMPLE

Cross-sectional photographs of the electroless nickel/palladium/goldplated layers obtained according to Example and Comparative Example wereobserved by a scanning electron microscope (SEM). Results of theobservation were shown in FIGS. 2 and 4.

Referring to FIG. 2, in the case of the nickel/palladium/gold platedlayer according to the related art, it may be appreciated that thenickel plated coating has a significant thick thickness. In this case, acurrent flows in the nickel layer of the plated layer while mainlyflowing along a surface in a high frequency band, thereby causing aproblem that electrical resistance increases due to a skin effect inwhich the current mainly flows in the nickel layer rather than an innerlayer of copper wiring.

However, referring to FIG. 4 showing a cross-sectional photograph of theelectroless nickel/palladium/gold surface treatment plated layerprepared according to the exemplary embodiment of the present invention,it may be appreciated that the nickel plated coating has a thicknessalmost similar to those of the palladium and gold plated coatings.Therefore, it is possible to solve the problem that the electricalresistance increases in the high frequency band due to a thick thicknessof the nickel plated coating according to the related art. Therefore, itis possible to improve electrical characteristics of the printed circuitboard including the electroless nickel/palladium/gold surface treatmentplated layer.

With the electroless surface treatment plated layer of the printedcircuit board according to the exemplary embodiment of the presentinvention, the thickness of the nickel plated coating is speciallyminimized to 0.02 to 1 μm, thereby making it possible to form anoptimized electroless Ni/Pd/Au surface treatment plated layer.

In addition, with the electroless surface treatment plated layer of theprinted circuit board according to the exemplary embodiment of thepresent invention, the thickness of the nickel coating is significantlyreduced, thereby making it possible to reduce a phenomenon in which acurrent flows in the nickel layer having high electrical resistancecaused by the skin effect phenomenon in which the current flows along asurface in a high frequency band when the electroless surface treatmentplated layer is connected to the external devices in the wire bondingscheme, such that the electrical resistance increases and the electricalsignal characteristics are deteriorated. That is, the thickness of thenickel coating is significantly reduced to allow a more current to flowin the cu layer having low electrical specific resistance in the highfrequency band, thereby making it possible to improve the electricalcharacteristics.

In addition, with the electroless surface treatment plated layer of theprinted circuit board according to the exemplary embodiment of thepresent invention, the nickel coating has a thin thickness, therebymaking it possible to suppress generation of an electrical bridge due toundesired abnormal Ni plating generation between pattern spaces at thetime of the electroless nickel plating on the copper plated layer havinga pattern space of 25 μm or less.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims. Accordingly, suchmodifications, additions and substitutions should also be understood tofall within the scope of the present invention.

1. An electroless surface treatment plated layer of an electrolessprinted circuit board, the electroless surface treatment plated layercomprising: nickel (Ni) plated coating, palladium (Pd) plated coating,gold (Au) plated coating, wherein each of the nickel, palladium, andgold plated coatings has a thickness of 0.02 to 1 μm, 0.01 to 0.3 μm,and 0.01 to 0.5 μm.
 2. The electroless surface treatment plated layeraccording to claim 1, wherein it is connected to external devices in awire bonding scheme.
 3. The electroless surface treatment plated layeraccording to claim 1, wherein 8 wt % or more of phosphorus (P) iscontained in the electroless nickel plated coating.
 4. The electrolesssurface treatment plated layer according to claim 3, wherein thephosphorus (P) is to prevent oxidation of the electroless nickel platedcoating.
 5. A printed circuit board comprising the electroless surfacetreatment plated layer according to claim
 1. 6. The printed circuitboard according to claim 5, wherein even though a frequency bandincreases, an electrical resistance value of the electroless surfacetreatment plated layer does not increase.
 7. The printed circuit boardaccording to claim 5, wherein plated layers formed as the electrolesssurface treatment plated layer of the printed circuit board have a spaceof 25 μm or less therebetween.
 8. A method for forming an electrolesssurface treatment plated layer of a printed circuit board, the methodcomprising: forming an electroless surface treatment plated layer bysequentially forming nickel (Ni) plated coating, palladium (Pd) platedcoating, gold (Au) plated coating, wherein each of the nickel,palladium, and gold plated coatings has a thickness of 0.02 to 1 μm,0.01 to 0.3 μm, and 0.01 to 0.5 μm.
 9. The method according to claim 8,wherein 8 wt % or more of phosphorus (P) is contained in the electrolessnickel plated coating.
 10. The method according to claim 9, wherein aphosphorus content in the electroless nickel plated coating is adjustedby lowering a pH of a Ni plating solution.
 11. The method according toclaim 9, wherein a phosphorus content in the electroless nickel platedcoating is adjusted by adding a sulfur compound additive to a nickelplating solution.
 12. The method according to claim 8, wherein theelectroless gold plated coating is formed in a substitution/reductiontype.